1. Field of the Invention
The present invention relates generally to three-dimensional (3D) computerized tomography (CT) and more specifically, to a method and apparatus for improving the computational efficiency of an exact cone beam reconstruction.
2. Description of the Background Art
Recently a system employing cone beam geometry has been developed for 3D imaging that includes a cone beam x-ray source and a 2D area detector. An object to be imaged is scanned, preferably over a 360.degree. angular range, either by moving the x-ray source in a scanning path about the object or by rotating the object while the source remains stationary. In either case, the area detector is fixed relative to the source, and relative movement between the source and object provides the scanning (irradiation of the object by the cone beam energy). The cone beam approach has the potential to achieve 3D imaging in both medical and industrial applications both rapidly and with improved dose utilization.
The 2D area detector used for 3D imaging generally has detector elements arranged in a 2D array of rows and columns. Available area detectors have generally been of large size and low quality, such as used with x-ray image intensifiers, or of small size and high quality. High cost and other factors have made large area 2D array detectors having high quality and high resolution, generally unavailable. In U.S. Pat. No. 5,390,112 entitled THREE-DIMENSIONAL COMPUTERIZED TOMOGRAPHY SCANNING METHOD AND SYSTEM FOR IMAGING LARGE OBJECTS WITH SMALL AREA DETECTORS issued Feb. 14, 1995, and hereby incorporated by reference, a cone beam CT system is disclosed in which an x-ray source following a spiral scan path is used to image a relatively long object, wherein the x-ray detector is much shorter than the object. The only height requirement for the detector is that it be longer than the distance between adjacent turns in the spiral scan of the x-ray source. As the cone beam source follows the scan path, the detector acquires many sets of cone beam projection data, each set representative of the x-ray attenuation caused by the object at each of the many source/detector positions along the scan path.
The cone beam projection data is then manipulated to reconstruct a 3D image of the object. The manipulation of the cone beam projection data is quite computationally complex and comprises:
1) conversion of the projection data to Radon derivative data. This may be generally be accomplished using the techniques described in U.S. Pat. No. 5,257,183 entitled METHOD AND APPARATUS FOR CONVERTING CONE BEAM X-RAY PROJECTION DATA TO PLANAR INTEGRAL AND RECONSTRUCTING A THREE-DIMENSIONAL COMPUTERIZED TOMOGRAPHY (CT) IMAGE OF AN OBJECT issued Oct. 26, 1993, hereby incorporated by reference, PA1 2) conversion of the Radon derivative data to Radon data at polar grid points using, for example, the technique described in U.S. Pat. No. 5,446,776 entitled TOMOGRAPHY WITH GENERATION OF RADON DATA ON POLAR GRID POINTS issued Aug. 8, 1995, also hereby incorporated by reference, and PA1 3) performing an inverse 3D Radon transformation of the Radon data at the polar grid points using well known techniques, such as those described in detail in the fore-noted U.S. Pat. No. 5,257,183 for reconstructing image data that when fed to a display provides a 3D CT image of the object.
In view of the above computationally complex image data processing, efforts are needed for reducing the complexity.
Prior U.S. Pat. No. 5,333,164 entitled METHOD AND APPARATUS FOR ACQUIRING AND PROCESSING ONLY A NECESSARY VOLUME OF RADON DATA CONSISTENT WITH THE OVERALL SHAPE OF THE OBJECT FOR EFFICIENT THREE DIMENSIONAL IMAGE RECONSTRUCTION, issued Jul. 16, 1994 discloses a technique for reducing the amount of computation needed to make a 3D cone beam CT image by a priori knowledge of the aspect ratio of the object being imaged for reducing the points in Radon space that are sampled. Although this technique reduces computationally complexity, it would be desirable to reduce the required computations at an earlier stage of the reconstruction processing.
Prior U.S. Pat. No. 5,390,226 entitled METHOD AND APPARATUS FOR PRE-PROCESSING CONE BEAM PROJECTION DATA FOR EXACT THREE DIMENSIONAL COMPUTER TOMOGRAPHIC IMAGE RECONSTRUCTION OF A PORTION OF AN OBJECT, issued Feb. 14, 1995 discloses a technique for reducing the amount of computation needed to make a 3D cone beam CT image by attempting to retain for further processing that cone beam attenuation data acquired within a select region on the surface of the detector that provides projection data corresponding to beams actually attenuated by passing through the object. Thus, unnecessary detector data is discarded at the earliest possible opportunity of the image processing. However, the technique of U.S. Pat. No. 5,390,226, as illustrated in FIG. 2 (b) therein, only masks the detector data, i.e, reduces the projection data, by limiting the data used for further processing to that data between upper and lower projections of the object on the detector, i.e., the upper and lower boundaries of the "shadow" of the object. The projection data between the left and right boundaries of the object shadow and the left and right boundaries of the detector are not so limited, and in fact are ignored and assumed to not exist.
Since a practical CT imaging system is designed to image an object having a given maximum width, when the object being imaged is less than the maximum width, when using the technique of U.S. Pat. No. 5,390,226 there will be no useful projection data between the left and right boundaries of the object shadow and the left and right boundaries of the detector. It would be desirable if means were provided to actually determine the width of the object shadow in order to properly reduce the amount of data being processed.
Furthermore, if the actual object being imaged is not symmetric, as is the case, for example, with a medical patient, as the source/detector moves about the scan path, the width of the shadow will vary, resulting in a variable shadow width. The technique of U.S. Pat. No. 5,390,226 assumes a fixed width for the object shadow, requiring that a maximum permissible width be used to prevent the generation of image artifacts.
An object of the present invention is to reduce the computational complexity of 3D cone beam image reconstruction at the earliest possible stage in the reconstruction processing.
It is a further object of the invention to provide such reducing computation in an adaptive manner, thereby maximizing the efficiency of the image reconstruction processing.